In a contemporary optical network system, a number of optical network nodes may be coupled to each other via optical fibers. Optical signals are transmitted across the optical fibers from one optical network node to another. The optical network nodes may be located in different geographical sites. All physical modules that comprise the node have a non-zero failure rate. In the event of a module failure, modules in it usually takes a significant amount of time for a technician to travel to a particular site to replace a failed or degraded optical network node. During this time, information cannot pass and the network is said to be “unavailable” for flows that are disrupted.
To improve the availability of the optical network nodes and to reduce potential down time due to equipment failure in an optical network system, some existing optical network nodes include one or more protection modules as a back up for one or more working modules. Optical switches gating an incoming optical signal to different parts of the optical network node is used in some optical network nodes to switch from a failed module to the protection module when an error signal is detected. However, the system may not know whether the protection module is operational until the switch passes the optical signal to the protection module when the working module fails. Therefore, a hidden failure problem with the protection module may exist without being detected until the protection module is used. Such a hidden failure problem may cause unexpected delay in the recovery of the optical network system.
Alternatively, some existing optical network nodes include optical splitters to split optical signals entering different parts of the optical network nodes. But such a hardware configuration typically adopts a fairly complex rerouting strategy to allow for the case in which only some of the parts of the optical network nodes fail and the optical network nodes are not entirely replaced with protection equipment.